The present invention relates generally to instrumentation equipment, and specifically to high-accuracy drivers for automatic testing equipment.
Present-day very large scale integrated (VLSI) circuits are routinely rated at operating frequencies of the orders of hundreds of MHz. Testing systems for these circuits must of necessity be able to switch significantly faster than the rate of the systems they are testing, so that the testing system is not a limiting factor in the testing process. The testing systems must also be able to apply accurate voltage levels to circuits being tested. Thus testing systems which are able to switch at high frequency rates of at least 1 GHz or even several GHz, and which are also able to accurately control the voltage swings of the signals, are necessary.
FIG. 1 is a schematic electronic diagram of a last stage 10 of an automatic test equipment (ATE) driver, as is known in the art, for producing signals comprising high-frequency controlled voltage swings. System 10 comprises a driver 12 and an external feedback circuit 14. Driver 12 receives opposite phase switching signals from a preamplifier 16. The preamplifier output signals are applied to the respective gates of differential pair transistors 18 and 20, comprised in driver 12, which have their emitters coupled together in an emitter coupled logic (ECL) stage. Transistors 18 and 20 generate opposite phase output signals OUT and OUTN at their collectors. Both outputs have a swing between controlled upper and lower levels as explained below.
The collector of a control transistor 26 is connected to the coupled emitters of transistors 18 and 20, so that transistor 26 acts to control the current through transistors 18 and 20, and thus the upper and lower levels of OUT and OUTN. The emitter of transistor 26 is connected in series with a reference resistor 22, and a reference voltage is measured across the resistor for use by feedback circuit 14.
Feedback circuit 14 comprises an operational amplifier 24, which reads the reference voltage generated across resistor 22 and feeds the voltage, via the inverting input of the amplifier, to the gate of control transistor 26. Amplifier 24 also receives at its non-inverting input a swing control voltage which sets the swing voltage, i.e., the peak-peak voltage, of signals OUT and OUTN.
Typically, some or all components of last stage 10 are built on a single chip, although some or all of the components may be off-chip and/or discrete components. Furthermore, each of transistors 18, 20, and 26 may be replaced by a respective plurality of transistors in parallel, in order to increase the current that can be passed in each of the respective paths. Alternatively or additionally, the emitter area of transistors in the path is increased so as to increase the current carrying capacity. Preferably, transistors 18, 20, and 26, or the respective pluralities replacing the transistors, are bipolar. However, the basic concepts of the operation of the last stage also apply if the transistors are CMOS transistors. Most preferably, the bipolar transistors are implemented in silicon-germanium, or other heterostructure technology. Preferably, amplifier 24 is an operational amplifier implemented from conventional bipolar or MOSFET transistors.
The accuracy of the feedback loop of systems such as last stage 10 is limited because intrinsic variations in parameters within driver 12 can only be indirectly sensed by the external feedback circuit. The variations, such as changes in current gain, base-emitter voltage, or modulation of the base width (the Early effect) of transistors 18, 20, and 26, can not be properly sensed by the external circuit.
In preferred embodiments of the present invention, an instrumentation driver comprises both a main driver and a mirror driver, preferably connected by a feedback amplifier. The main driver receives an input alternating signal, and generates a corresponding alternating output signal. The mirror driver receives a substantially fixed voltage, and generates a corresponding, approximately-fixed output signal. The mirror driver is subject to substantially the same intrinsic variations in operating conditions and voltage levels as is the main driver. The mirror driver effectively senses these variations and cooperates with the feedback amplifier to stabilize the output alternating signal of the main driver. Thus, the instrumentation driver achieves significantly higher accuracy in its high-speed, alternating output signals than do instrumentation drivers known in the art.
The mirror driver is implemented to have electrical properties substantially similar to those of the main driver, and is maintained in the same operating environment as the main driver. Preferably, at least some stages of the main driver and corresponding stages the mirror driver are implemented using substantially the same numbers of corresponding elements. The approximately-fixed output signal from the mirror driver is used as an input to the feedback amplifier, so that the mirror driver and the amplifier together comprise a feedback path or the main driver.
Since the main driver and the mirror driver have substantially similar electrical properties and are in the same environment, variations in parameters of the main driver and variations in corresponding parameters of the mirror driver will be substantially similar. Since the mirror driver is in the feedback path, variations in the main driver, which do not directly show in the feedback path of prior art instrumentation drivers, are directly incorporated into the feedback path of preferred embodiments of the present invention. These factors contribute to the high accuracy of output signals that the present instrumentation driver achieves.
In some preferred embodiments of the present invention, the main driver and mirror driver are both driven by substantially similar preamplifiers operating in the same environment, so that variations in corresponding parameters of the preamplifiers are also substantially similar. The main driver preamplifier receives an alternating signal from an external source, and generates a corresponding alternating signal as an input to the main driver. The mirror driver preamplifier receives a substantially fixed voltage, and generates, as an input to the mirror driver, a corresponding approximately fixed voltage which reflects changes in the environment of the driver preamplifier.
In some preferred embodiments of the present invention, the mirror driver preamplifier is a simplified version of the main driver preamplifier. The simplified mirror driver preamplifier duplicates the conditions at the output of the main driver preamplifier.
There is therefore provided, according to a preferred embodiment of the present invention, instrumentation driver apparatus, including:
a main driver, coupled to receive an alternating input signal and having a main circuit structure, which is adapted to generate, responsive to the alternating input signal, a main output signal with alternating voltage;
a mirror driver, coupled to receive a direct voltage input and having a mirror circuit structure located in proximity to the main circuit structure, and adapted to generate a mirror output signal responsive to the direct voltage input, such that a variation in an operating condition of the main driver causes a corresponding variation in the mirror output signal; and
a feedback circuit, coupled to receive the mirror output signal and to provide, responsive thereto, a feedback stabilization input to the main driver so as to stabilize the main output signal.
Preferably, the feedback circuit includes an amplifier which is coupled to receive a swing control voltage and to vary the main output signal responsive thereto.
Further preferably, the feedback circuit is coupled to provide the feedback stabilization input to the mirror driver.
Preferably, the main circuit structure includes a plurality of main elements, and the mirror circuit structure includes a corresponding plurality of mirror elements coupled in a substantially similar manner to the plurality of main elements included in the main circuit structure.
Preferably, the apparatus includes:
a main driver preamplifier which supplies the alternating input voltage to the main driver; and
a mirror driver preamplifier which supplies the direct voltage input to the mirror driver, wherein the main driver preamplifier is implemented in a substantially similar environment to that of the mirror driver preamplifier, such that a variation in an operating condition of the main driver preamplifier causes a corresponding variation in the mirror output signal.
Further preferably, the main driver preamplifier includes a plurality of stages including a main power output stage, and the mirror driver preamplifier includes a mirror power output stage substantially similar in number of elements and coupling of the elements to the main power output stage.
Further preferably, the mirror driver preamplifier includes a plurality of mirror elements coupled in a substantially similar manner to coupling of a corresponding plurality of main elements included in the main driver preamplifier.
Further preferably, the main driver preamplifier is coupled to receive an overshoot feedback input in order to limit an overshoot in the main output signal.
Preferably, the main output signal includes signals having frequencies greater than approximately 1 GHz.
Preferably, the main output signal includes substantially rectangular signals having a transit time between an upper and a lower level less than approximately 200 ps.
Preferably, the main output signal includes substantially rectangular signals including an upper level and a lower level having an accuracy of the order of 10 mV or less.
Preferably, the main circuit structure includes a main differential pair of transistors which provide the main output signal at a collector of one of the pair of transistors.
Further preferably, the mirror circuit structure includes a mirror differential pair of transistors having substantially similar characteristics to the main differential pair of transistors.
Further preferably the main differential pair of transistors include respective pluralities of transistors coupled in parallel.
Further preferably, the mirror circuit structure includes a non-differential transistor which is coupled in a substantially similar manner and which has substantially similar characteristics to one of the plurality of transistors coupled in parallel.
Preferably, the apparatus is constructed so that at least some elements of the mirror circuit structure operate in a substantially similar environment to that of at least some elements of the main circuit structure.
Further preferably, the environment includes a single chip containing the at least some elements of the mirror circuit structure together with the at least some elements of the main circuit structure.
There is further provided, according to a preferred embodiment of the present invention, instrumentation driver apparatus, including:
a first main driver, coupled to receive a first alternating input signal and having a first main circuit structure, which is adapted to generate, responsive to the first alternating input signal, a first main output signal with alternating voltage;
a first mirror driver, coupled to receive a first direct voltage input and having a first mirror circuit structure located in proximity to the first main circuit structure, and adapted to generate a first mirror output signal responsive to the first direct voltage input, such that a variation in an operating condition of the first main driver causes a corresponding variation in the first mirror output signal;
a first feedback circuit, coupled to receive the first mirror output signal and to provide, responsive thereto, a first feedback stabilization input to the first main driver so as to stabilize the first main output signal;
a second main driver, coupled to receive a second alternating input signal and having a second main circuit structure substantially similar to the first main circuit structure, which is adapted to generate, responsive to the second alternating input signal, a second main output signal with alternating voltage;
a second mirror driver, coupled to receive a second direct voltage input and having a second mirror circuit structure substantially similar to the first mirror circuit structure, located in proximity to the second main circuit structure, and adapted to generate a second mirror output signal responsive to the second direct voltage input, such that a variation in an operating condition of the second main driver causes a corresponding variation in the second mirror output signal; and
a second feedback circuit, coupled to receive the second mirror output signal and to provide, responsive thereto, a second feedback stabilization input to the second main driver so as to stabilize the second main output signal, so that the first main driver, the first mirror driver, and the first feedback circuit, are electrically independent of the second main driver, the second mirror driver, and the second feedback circuit, and so that the first main output signal and the second main output signal are combined to form a tri-level output.
Preferably, the apparatus includes a power supply which powers the first main driver, the first mirror driver, the first feedback circuit, the second main driver, the second mirror driver and the second feedback circuit.
Alternatively, the apparatus includes:
a first power supply which powers the first main driver, the first mirror driver, the first feedback circuit; and
a second power supply which powers the second main driver, the second mirror driver and the second feedback circuit.
There is further provided, according to a preferred embodiment of the present invention, a method for generating a signal, including:
generating, in a main driver having a main circuit structure, a main output signal with alternating voltage, responsive to an alternating input signal;
providing a mirror driver, coupled to receive a direct voltage input and having a mirror circuit structure located in proximity to the main circuit structure;
generating a mirror output signal in the mirror circuit structure responsive to the direct voltage input such that a variation in an operating condition of the main driver causes a corresponding variation in the mirror output signal; and
providing a feedback stabilization input to the main driver responsive to the mirror output signal so as to stabilize the main output signal.
Preferably, providing the feedback stabilization input includes providing a feedback amplifier which receives a swing control voltage and which varies the main output signal responsive thereto.
Preferably, the mirror circuit structure includes a plurality of mirror elements coupled in a substantially similar manner to a corresponding plurality of main elements comprised in the main circuit structure.
Preferably, the method includes:
supplying the alternating input voltage to the main driver from a main driver preamplifier;
supplying the direct voltage input to the mirror driver from a mirror driver preamplifier, and
implementing the main driver preamplifier in a substantially similar environment to that of the mirror driver preamplifier, so that a variation in an operating condition of the main driver preamplifier causes a corresponding variation in the mirror output signal.
Further preferably, the main driver preamplifier includes a plurality of stages including a main power output stage, and the mirror driver preamplifier includes a mirror power output stage substantially similar in number of elements and coupling of the elements to the main power output stage.
Further preferably, providing the feedback stabilization includes coupling the main driver preamplifier to receive an overshoot feedback input so as to limit an overshoot in the main output signal.
Preferably, generating the main output signal includes generating signals comprising frequencies greater than approximately 1 GHz.
Preferably, generating the main output signal includes generating substantially rectangular signals having a transit time between an upper and a lower level less than approximately 200 ps.
Preferably, generating the main output signal includes generating substantially rectangular signals including an upper level and a lower level having an accuracy of the order of 10 mV or less.
Preferably, the main circuit structure includes a main differential pair of transistors, and the mirror circuit structure includes a mirror differential pair of transistors having substantially similar characteristics to the main differential pair of transistors.
Further preferably, the main differential pair of transistors include respective pluralities of transistors coupled in parallel, and the mirror circuit structure includes a non-differential transistor which is coupled in a substantially similar manner and which includes substantially similar characteristics to one of the plurality of transistors coupled in parallel.
Preferably, providing the mirror driver includes operating at least some elements of the mirror circuit structure in a substantially similar environment to that of at least some elements of the main circuit structure.
Further preferably, the environment includes a single chip.
The present invention will be more fully understood from the following detailed description of the preferred embodiments thereof, taken together with the drawings, in which: